If white plays a, black plays b.. And still alive..
a and b are miai.. If black takes one, white plays the other and kills..
The main trap in this problem is that Black cannot play at . Because
is a snapback, in that if Black plays at a to capture the
stone, then White will capture the Black stones by playing at
again. Hence, the
stones cannot be saved, and the whole group is dead.
Because white can kill! Black can't play at a! (It'd be self-atari..)
Solution by Reuven.